The present invention relates generally to resonant type DC-DC converters. More particularly, the present invention relates to resonant type DC-DC power converters implemented in LED drivers and having capacitive mode protection circuitry and programming.
Referring to FIG. 1, an exemplary constant power LED driver 100 as conventionally known in the art may often implement a resonant type DC-DC converter 102 because of its relatively high efficiency and variable gain. An output current from the DC-DC converter 102 passes through a load RL, such as an LED array coupled across output terminals for the driver. A current sensor 106 such as a current sensing resistor R6 feeds back the output current information to a controller 104, which is configured to control the output current to a reference current set point defined by input signal I_ref. The controller accomplishes this is many cases by regulating the driven frequency of switching elements in the DC-DC converter via control signals CTRL to maintain the target current set by the reference current I_ref.
The relatively high efficiency of DC-DC resonant type converters 102 is provided at least in part via soft switching, or zero-voltage switching (ZVS), of the associated switching elements. As used herein, the term ZVS refers to on/off transitions for respective switching elements only during a time period when there is no voltage across them.
However, one notable problem with such implementations as described above is that if the operating frequency is too low, the DC-DC resonant converter could enter into reverse-recovery switching, or capacitive mode switching, that instantly damages associated circuitry. The operating frequency of the DC-DC converter switches must be higher than the associated resonant frequency to maintain inductive switching (i.e., soft-switching).
As illustrated in FIG. 2, the gain of the converter output changes in relation to the operating frequency. For example, a first output voltage curve Vout_1 and a first output current curve Iout_1 are demonstrated as a function of frequency in association with a high load impedance. A second output voltage curve Vout_2 and a second output current curve Iout_2 are demonstrated as a function of frequency in association with a low load impedance. A resonant frequency of the DC-DC converter tank for the high output impedance load, f_res1, is higher than the resonant frequency of the tank, f_res2, for the lower output impedance load. The steady state frequency f_op1 for the high load is greater than the resonant frequency f_res1, and the steady state frequency f_op2 for the lower load is greater than the resonant frequency f_res2, so that in either condition soft-switching can be obtained.
The controller 104 will typically have a minimum operating frequency setting, f_min. Again by reference to FIG. 2, the minimum operating frequency f_min is designed to be less than the steady state frequency f_op2 to guarantee normal operation for the low impedance load (curve 2).
When the driver 100 is powered on, the controller 104 will quickly sweep the operating frequency from a maximum setting toward the minimum setting f_min to provide the target current set by the reference I_ref. However, the control loop in the controller has an inherent delay, especially for proportional-integral (PI) implementations. Accordingly, the controller will typically sweep the operating frequency all the way down to the minimum frequency f_min, and then back up to the steady-state frequency f_op. In the case of a higher impedance load, the resonant tank will therefore go through capacitive mode switching because f_min<f_res1. This could be a very harmful operating condition if there is no hardware protection circuit employed to avoid capacitive mode switching.
Capacitive mode protection circuitry is known in the art as a supplement to avoid capacitive mode switching, but such circuitry is typically costly, and may increase the product cost to an unacceptable level. Therefore, it would be desirable to implement lower-cost solutions including software control algorithms to avoid this harmful capacitive mode operating.